In the past, workers in the fields of metallurgy and welding have attempted to prevent cracking in metallic objects. Prevention of cracking is particularly important in fields where containment is a major factor, such as the nuclear field and, to a lesser extent, the chemical and biotechnology fields. The prevention of cracking is of particular concern in connection with-pipe weld joints.
Intergranular stress corrosion cracking in metal objects, such as austenitic stainless steel pipes, typically occurs after three conditions have been fulfilled. First, the material must be susceptible to cracking, such a metal or alloy having grain-boundary carbide precipitates. Second, a portion of the metal object is under tension, such as that resulting from tensile residual stress. Third, the object is exposed to a crack-promoting environment, such as a corrosive environment or a crevice formed near the tensile-stressed area.
Because tensile stress is associated with cracking, most of the effort in the nuclear field has been concentrated on producing objects, such as pipe weld joints, which are under compressive stress. One method of producing a joint under compressive stress is to provide welding heat around the outside diameter of the pipe (at least during the last few welding passes) while cooling the interior of the pipe, such as using water. The result is a pipe weld joint which is substantially under compressive stress.
In connection with studying the cracking problem, many testing techniques have been developed. However, such testing techniques are typically hampered because of a lack of knowledge of (1) the location of tensile residual stress, or (2) the location of latent cracks. An example of such hampered testing is the technique known as "reversing D.C. crack growth monitoring," such as that discussed in "Environmental Crack Growth Measurement Techniques," a final report prepared by General Electric Co. for the Electric Power Research Institute, Report No. NP-2641, November, 1982, incorporated herein by reference. In a General Electric Nuclear Energy Company study, a predefected "dogbone" shape specimen was used for investigation. All tests were conducted on the dogbone specimens in oxygenated water in a stainless steel autoclave at 1.03.times.10.sup.7 Pa (1500 psi) and 288 degrees Celsius. One reference and six active potential probe pairs were attached to each specimen. The specimens were subjected to uniaxially cyclic or static loads. Two defect shapes were employed. Each was an arc of a circle, one with a radius of 0.635 mm ( 0.025 inch) and the other with a radius equal to 1.59 mm (0.0635 inch). All defects were intended to be 0.625 mm (0.025 inch) deep. Each defect was introduced by electrical discharge machining using a single shaped electrode. After a crack had propagated to the desired dimensions and the specimen removed from the autoclave, it was fractured in the plane of the crack so that the dimensions of the crack could be measured for comparison with dimensions derived from potential drop measurements.
The section of 10.2 cm (4 inch) ID pipe that was used to demonstrate the feasibility of the reversing dc potential technique to crack measurement in components was defected by electrical discharge machining. In the pipe, as in the specimens, the aim was to position the defect and probes as accurately as possible, but in the pipe it was necessary not only to measure the distance from the end of the pipe but also to accurately space the probes circumferentially on the ID surface. According to this technique, one or more electrode pairs are positioned in the vicinity of a crack or latent crack, and reversing direct current is applied. Analysis of measured electrical characteristics provides an indication of the rate of growth of cracks. As noted, however, the electrodes are positioned adjacent the cracks or latent cracks. In a pipe, the electrodes are preferably axially positioned within about 0.15 inches of the cracks. Accordingly, these tests are hampered in situations where the location of the cracks or latent cracks, or the location of residual tensile stress, is unknown.
Another situation in which testing is hampered is in the laboratory testing of equipment which is intended for testing under bending loads. Such laboratory testing is best conducted using samples which have tensile residual stresses in known locations.
One method which has been attempted in order to produce localized cracking is to subject a metallic object to crack-promoting stress, such as a corrosive environment, only over a portion of that metallic object. However, because of the above-noted relationship of tensile residual stress, such a technique will be successful only where the localized crack-promoting stress is applied to an area of the metal object which is under tensile stress. When the location of tensile residual stress is unknown, such a technique has only a hit-or-miss success.